Semiempirical molecular orbital calculations at the AM1 and PM3 levels have been used to model stable free-radical-mediated living polymerization reactions. These calculations predict that in the living free-radical polymerization, the reversible terminator of the growing polymer radical must have a calculated bond dissociation enthalpy of less than 35 kcal/mol in order to achieve reasonable rates of chain propagation. Furthermore, the calculations also correctly predict that reversible terminators such as 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) have an endothermic enthalpy of reaction with styrene monomer and, therefore, are not able to initiate new chains, a prerequisite for narrow polydispersity in these systems. Calculations have been performed on 21 possible reversible terminator structures. One of these reversible terminators, di-tert-butyl nitroxide, was predicted to have a lower bond dissociation enthalpy than the benchmark TEMPO reversible terminator. Experiments have confirmed that under comparable reaction conditions, the di-tert-butyl nitroxide-mediated reaction proceeds to completion more rapidly than the comparable TEMPO reaction, consistent with the predictions of the molecular orbital calculations.